Metal detectors



April 19, 1955 F. w. BUBB, JR

METAL DETECTORS 2 Sheets-Sheet 1 Filed July 14, 1952 FIG. 2.

I H 4-H v INVENTOR. FRANK W. Buss, JR.

Arm/my April 19, 1955 Filed July 14. 1952 F. W. BUBB, JR

METAL DETECTORS 2 Sheets-Sheet 2 INVENTOR. FRANK W. BuBB,JR.

United States Patent METAL DETECTORS Frank W. Bubb, Jr., St.

L. Sprengnether, F. Sprengnether,

Louis, Mo., assignor to Roberta administratrix of the estate of William deceased Application July 14, 1952, Serial No. 298,799 Claims. (Cl. 324-41) object of the invention being to provide a metal detector of the type and for the purpose referred to above which because of its improved construction and arrangement is capable of performing its intended function in an improved and highly etficient manner.

Fig. l is a front elevation of a portion of the improved metal detector of this invention, parts thereof being broken away to reveal structure portions which otherwise would be concealed.

Fig. 2 is a fragmentary, vertical section taken on line 2-2 of Fig. 1.

Fig. 3 is a horizontal section taken on line 3--3 of Fig. 1.

Fig. 4 is a wiring detail showing the manner of electrically connecting the various elements of which the improved metal detector is comprised.

In the drawings, wherein is shown for purposes of illustration, merely, one embodiment of the invention, A designates a yoke which is provided with coils 1 and 2 that are disposed vertically at opposite sides of the yoke A. The coils 1 and 2 are shown in the drawings as being mounted on a supporting structure 3 of suitable construction and arrangement, there being at the upper ends of said coils 1 and 2 a horizontal bar 4 and said bar 4 is secured adjacent to its opposite ends to the cores 1' and 2 of the respective coils 1 and 2, such attachment of the opposite end portions of the upper bar to the cores of the coils 1 and 2 being accomplished with the aid of suitable fastening devices 5. In like manner a lower bar 6 is provided whose opposite end portions are secured, with the aid of suitable fastening devices 7, to the cores 1 and 2 of the coils at the lower ends thereof. The bars 4 and 6 at the upper and lower ends of the coils 1 and 2 are formed of material suitable for the purpose, steel, for instance, and the lower bar 6 has fixed thereto at the top surface thereof a plate 8 which may likewise be formed of steel. The plate 8 is secured to the bar 6 by means of fastening devices 9 and opposite end portions of said plate are supported by and are fastened to stringers 10 that form parts of the supporting structure 3, the attachment of the plate 8 to the stringers 10 being accomplished with the aid of fastening devices 11. The top wall of the supporting structure is completed by panels 12 formed of wood or other suitable material, said panels being supported by portions of the stringers 10 and by end stringers 10'. By referring to Figs. 1, 2, and 3 it will be noted that the coils 1 and 2 and the upper bar 4 are housed within a suitable housing 13.

The coils 1 and 2 are so wound that the magnetic field in the center of the yoke from top to bottom is substantially uniform, the coils being wound so that the M. M. F. of one opposes that of the other, with the result that the flux passes upwardly of the two sides of the yoke but downwardly through the air gap of the yoke from the upper, horizontal bar 4 to the lower, horizontal bar 6. the flow of flux being roughly indicated by line in Fig. 4. Now, when a small piece of paramagnetic material is placed in the field, the reluctance of the fiux path is decreased slightly causing an increase in flux for the same M. M. F. Thus, when material containing a small piece of paramagnetic material is thrust through the yoke there is a sudden change in the magnitude of the flux LII ' and between the upper and in the yoke sides. Also, wound on each yoke side is a pickup coil 15, so that the changing flux threading each coil induces an E. M. F. in the coi The coils are connected in series so that the induced E. M. F.s are added and the total B. M. F. of the two pickup coils is applied to the tube circuit (Fig. 4).

This tube circuit consists of a modulator 17 which accepts the more or less slowly varying voltage output of the pickup coils 15 and converts it to a sixty cycle voltage whose amplitude varies in accordance with the pickup coil voltage, this being accomplished by means of a converter 18 and transformer 19. The double-triode circuit, shown in Fig. 4, comprises a conventional A. C. amplifier 20 with gain adjustable by a potentiometer 21. The A. C. output of the amplifier is applied to one side of a neon tube 22 which is normally extinguished, the other side of the neon tube being connected through resistors 23 to the grid of a thyratron tube 24. With respect to the thyratron circuit, an A. C. voltage is applied to the plate, while an A. C. voltage, 180 degrees out of phase with the plate voltage, is applied through resistance to its grid. The thyratron tube is normally extinguished in the absence of a signal from the A. C. amplifier 20; however, when the voltage across the neon tube becomes large enough due to an output of the A. C. amplifier, this tube conducts, increasing the voltage at the grid of the thyratron tube 24, causing it to conduct current through a relay coil in its plate. This causes the relay 25, shown in Fig. 4, to operate, the connectors being arranged to extinguish the neon tube and sound an alarm, indicating that magnetic material has been passed through the yoke.

I claim:

1. A metal detector which includes a yoke comprising a pair of opposed coils having cores, means for electrically connecting said coils to each other, a source of potential to which said coils are electrically connected, an upper bar of magnetic material connected at its opposite ends to the upper end portions of the cores of said coils, a lower bar of magnetic material connected at its opposite ends to the lower end portions of said cores of said coils, said upper bar and said lower bar being formed of magnetic material which does not possess as good magnetic properties as the cores of the coils, said coils being so wound that the magnetic field between the coils lower bars of material is substantially uniform and with the M. M. F. of one coil opposing that of the other coil so as to cause the flux to pass in one direction of the two sides of the yoke and in the opposite direction through the air gap of the yoke from one bar of material to the other bar of material of said yoke, and a plate arranged in contact with and secured to said lower bar which tends to spread the effective field of the metal detector.

2. A metal detector which includes a yoke comprising a pair of opposed coils having cores, means for electrically connecting said coils to each other, a source of potential to which said coils are electrically connected, an upper bar of magnetic material connected at its opposite ends to the upper end portions of the cores of said coils, a lower bar of magnetic material connected at its opposite ends to the lower end portions of said cores of said coils, said upper bar and said lower bar being formed of magnetic material which does 'not possess as good magnetic properties as the cores of the coils, said coils being so Wound that the magnetic field between the coils and between the upper and lower bars of material is substantially uniform and with the M. M. F. of one coil opposing that of the other coil so as to cause the flux to pass in one direction of the two sides of the yoke and in the opposite direction through the air gap of the yoke from one bar of material to the other bar of material of said yoke, a plate arranged in contact with said lower bar which tends to spread the effective field of the metal detector, and a housing for enclosing said coils and said upper bar.

3. A metal detector which includes a yoke comprising a pair of opposed coils having cores, means for electrically connecting said coils to each other, a source of a potential to which said coils are electrically connected,

an upper bar of magnetic material connected at its opposite ends to the upper end portions of the cores of said coils, a lower bar of magnetic material connected at its opposite ends to the lower end portions of said cores of said coils, said upper bar and said lower bar being formed of magnetic material which does not possess as good magnetic properties as the cores of the coils, said coils being so wound that the magnetic field between the coils and between the upper and lower bars of material is substantially uniform, a supporting structure for supporting said coils and said housing, and a plate arranged in contact with said lower bar which tends to spread the effective field of the metal detector.

4. A metal detector which includes a yoke comprising a pair of opposed coils having cores, means for electri-- cally connecting said coils to each other, a source of potential to which said coils are electrically connected, an upper bar of magnetic material connected at its opposite ends to the upper end portions of the cores of said coils, a lower bar of magnetic material connected at its opposite ends to the lower end portions of said cores of said coils, said upper bar and said lower car being formed of magnetic material which does not possess as good magnetic properties as the cores of the coils, said coils being so wound that the magnetic field between the coils and between the upper and lower bars of material is substantially uniform, a supporting structure for supporting said coils and said housing, in the upper portion of which said lower bar is disposed, and a plate arranged in contact with said lower bar which tends to spread the effective field of the metal detector.

5. A metal detector which includes a yoke comprising a pair of opposed coils having cores, means for electrically connecting said coils to each other, a source of potential to which said coils are electrically connected, an upper bar of magnetic material connected at its opposite ends to the upper end portions of the cores of said coils, a lower bar of magnetic material connected at its opposite ends to the lower end portions of said cores of said coils, said upper bar and said lower bar being formed of magnetic material which does not possess as good magnetic properties as the cores of the coils, said coils being so wound that the magnetic field between the coils and between the upper and lower bars of material is substantially uniform and with the M. M. F. of one coil opposing that of the other coil so as to cause the flux to pass upwardly of the two sides of the yoke and downwardly through the air gap of the yoke from the upper bar of material to the lower bar of material, of said yoke, a plate arranged in contact with said lower bar, a housing of inverted U-shaped configuration for enclosing said coils and said upper bar, and a plate arranged in contact with said lower bar which tends to spread the effective field of the metal detector.

References Cited in the file of this patent UNITED STATES PATENTS 2,228,293 Wurzbach Jan. 14, 1941 2,489,920 Michel Nov. 29, 1949 2,513,745 Reynolds July 4, 1950 2,587,664 Stout Mar. 4, 1952 2,614,151 Gilson et al Oct. 14, 1952 

